A corollary to the debate about mass incarceration is the one about prisoner re-entry, which doesn’t get the attention it deserves even as the problem has escalated.

In 1980, state and federal prisons released fewer than 170,000 inmates each year. Today, the number is about 650,000, or roughly the population of Boston. Much of the focus in the popular press is on the number of people incarcerated, but the vast majority of people under correctional supervision are not behind bars. Instead, they’re living in the community while on parole or probation. As of 2015, 6.7 million people were under the care of the correctional system, but only 2.1 million—less than a third—were physically in custody.

That ratio hasn’t changed much over the past 30 years, and neither has the fact that ex-offenders are a major source of criminal behavior. About two-thirds of the people freed from prison commit new crimes, and the majority of all prison admissions each year comprises individuals who violated the conditions of their probation or parole.

Justice Department studies from the 1990s revealed that 43% of ex-felons on probation were rearrested within three years, and half of the arrests were for a violent crime or drug offense. Similarly, 67% of parolees were rearrested within three years for a felony or serious misdemeanor, and more than half were back in prison. Even prisoners considered “nonviolent” didn’t all stay that way after being released. Nearly 22% were eventually rearrested for violent crimes that included assault, rape and murder. A quarter-century later, these disturbing rates of recidivism continue.

“Overall, 67.8% of the 404,638 state prisoners released in 2005 in 30 states were arrested within 3 years of release, and 76.6% were arrested within 5 years of release,” according to a Justice Department analysis published in 2014. “Among prisoners released in 2005 in 23 states with available data on inmates returned to prison, 49.7% had either a parole or probation violation or an arrest for a new offense within 3 years that led to imprisonment, and 55.1% had a parole or probation violation or an arrest that led to imprisonment within 5 years.”

The persistence of recidivism is no great mystery. The majority of ex-convicts return to crime-plagued communities and re-establish relationships with other people leading dysfunctional lives and engaged in antisocial behavior. Re-entry programs are designed to help them deal with this environment, stay out of trouble and support themselves as law-abiding citizens. Many of these programs are small, and some get better results than others. The good ones deserve more attention from our policy makers with an eye toward funding and replicating what works.

Jon Ponder’s Hope for Prisoners program, based in Las Vegas, has been in operation since 2009 and serves more than 250 ex-offenders annually. A 2016 analysis of Hope conducted by researchers at the University of Nevada, Las Vegas, found that 64% of those who completed the job-readiness training course had found stable employment and that only 6% were reincarcerated.

Texas’ Prison Entrepreneurship Program (PEP) and Pennsylvania’s Peerstar program also provide job training, housing and life-skills training. According to a 2015 paper by the Texas Public Policy Foundation, between 5% and 7% of PEP participants recidivate within three years, and Peerstar’s re-entry program, which focuses on individuals with mental health problems, has reduced recidivism among mentally ill ex-offenders by 65%.

Robert Cherry, an economics professor at Brooklyn College whose research focuses on race and poverty, says work-readiness programs aimed at teaching basic, industry-specific job skills seem to be more effective than funneling former inmates into community colleges, which is popular in states like New York. In a new Manhattan Institute report on re-entry strategies which Mr. Cherry co-authored with Mary Gatta, a sociologist, they conclude that for many ex-cons, certificate programs “designed to prepare people for employment as soon as possible may be the best choice.”

“This is what President Obama’s My Brother’s Keeper initiative was about—getting more people vocational training,” Mr. Cherry told me. “But he was really slammed by the liberal left. They said he’s setting people up for crappy jobs.” However, given the education level of the average inmate—most are high school dropouts—this is a population that is more likely to make it through a short-term training program than through college-level remedial coursework. “There are studies that show certification programs are effective and reduce recidivism but there’s been a lack of interest among the liberal professorial class,” he said. “They think if we only provide enough support resources, everyone has a reasonable chance of getting a four-year degree.”

If liberals need to start backing what works in practice, their tough-on-crime counterparts must grapple with the reality that almost everyone in prison eventually gets out. Neglecting the welfare of ex-offenders will only facilitate more mayhem.